In particular, the requirement for speed increasing transmissions for wind turbines stems from their low rotational speed compared with the preferred generator speed, typically 1500 rpm. The low turbine speed is dictated by the fact that wind energy generation is a function of the turbine-swept area and the blade tip speed limits. Thus, the higher the power, the lower the rotor speed. In effect, power is directly proportional to the rotor diameter squared, while rotor speed is inversely proportional to tip diameter and/or the square root of rotor power; e.g. a 3000 kW turbine would run at 16 rpm compared with 44 rpm for a 400 kW machine with the same tip speed. Since rotor weight and torque are directly proportional to the rotor diameter cubed; bigger turbines not only have bigger transmission step up ratios but even bigger input torques and therefore a lower power to weight ratio. For example, whereas a 3000 kW turbine generates 7.5 times the power of a 400 kw machine, its torque and weight are increased by a factor of 20.54 (i.e. 7.5 raised to the power of 1.5), while its ratio is increased by a factor of 2.74 because its speed is reduced by the square root of 7.5.
Since the volume, weight and price of a gearbox is governed by its torque and overall ratio, there is an incentive to reduce weight by reducing the parasitic transient overload torques, which typically occur (and for which provision is made) in all fixed ratio wind turbine transmissions. These are created by the stochastic variations in wind speed, air density and unit aerodynamic energy over the large swept area of the turbine. Such variations lead to speed fluctuations of the turbine rotor hub at its input to the gearbox, which may occur many times per revolution. This is further complicated, firstly by steady changes in wind speed and secondly by the sudden changes which occur during gusts. Since wind energy is directly related to air velocity cubed, a 50% transient speed increase will increase aerodynamic power by a factor of 3. While some of this power will be dissipated by lower efficiency and some by the increased speed and kinetic energy in the turbine, it follows that with a fixed ratio transmission additional torques will be generated in trying to accelerate the generator. This stems from the fact that the polar moment of inertia of a generator about its own axis when referred to the axis of the turbine rotor is multiplied by the square of the step up ratio. Thus a generator requiring a step up ratio of 80/1 would have a referred inertia 6400 times that about its own axis. A 1-degree angular deviation of the turbine rotor hub from its mean speed of rotation would therefore suggest an 80 degree fluctuation of the generator over the same time scale.
An asynchronous generator can employ power conditioning to produce a smooth electrical output, but this masks the problem because mechanical acceleration torques are still required to change its speed. Such transient acceleration torques can only be mitigated by strain energy in the mechanical transmission path and so more rigid, transmissions will have higher torques.
The variable ratio transmission obviates this problem by changing its ratio in a complementary way, at the same rate as the transient change in the turbine speed. By so doing, generator torque, speed and phase angle are kept constant by allowing the turbine to accelerate and absorb the transient excess of power in the form of kinetic energy.
Prior variable ratio gearboxes for example as described in the present applicant's prior patent application WO 2004/109157, have been employed for applications including wind power turbines. However, their complication, for example, the need to use consumable components such as clutches and alternative power routes, increases their size, weight and manufacturing costs. The inventor has realised that a simple variable ratio gearbox is required which, in embodiments, provides generator speed control and a simple design.